The present invention relates to the determination of the depolarization efficiency of the atmosphere, using the dependence of photoexcited emission (photoluminescence emission, Raman scatter, or diffuse reflection) of a reference object. Raman scattering is the inelastic scattering of a photon. When light is scattered from an atom or molecule, most photons are elastically scattered (Rayleigh scattering), such that the scattered photons have the same energy (frequency) and wavelength as the incident photons. However, a small fraction of the scattered light (approximately 1 in 10 million photons) is scattered by an excitation, with the scattered photons having a frequency different from, and usually lower than, the frequency of the incident photons. In a gas, Raman scattering can occur with a change in vibrational, rotational or electronic energy of a molecule (see energy level). Chemists are concerned primarily with the vibrational Raman Effect. Depolarization efficiency is the fractional change in degree of polarization per unit length for electromagnetic radiation propagating through an environment, such as the atmosphere. Measurement of depolarization efficiency would improve imaging of remote objects through environments. The environments include atmospheric environments and underwater environments.
Depolarization of scattered laser radiation has been measured remotely using depolarization lidar. Depolarization lidar has been used to study nonspherical dust particles in the atmosphere, and multiscattering in clouds. In depolarization lidar measurements, a laser beam is emitted from the lidar system, passes through the atmosphere, backscatters off an atmospheric particle, and passes through the atmosphere again, and is collected in a detection device in the lidar which determines its polarization state. The scattering processes that affect the depolarization measured by lidar include both forward scattering processes and backscattering processes. However, the radiation that comes back to the lidar system has to be backscattered from an atmospheric particle at least once, and the impact of this single scattering process may or may not be greater than the forward scattering processes that depolarize the laser beam. The depolarization of radiation in the atmosphere biases polarimetric sensors. The application of depolarization lidar would be extended if the forward and backward scattering processes could be separately determined.
Some methods that may discriminate forward scattering processes from backscattering process requires either a one way path for the laser beam or a well characterized reflector a known distance from the laser and receiver. Both types of measurements are biased by receivers more sensitive to one polarization state over the others.
One way methods have the disadvantage that both receiver and transmitter must be at a large distance from each other. Methods where the receiver and the laser are near each other have required a well characterized reflector and a known distance from the reflector. Well characterized types of reflector include retroreflector and dielectric mirror. However, reflectors often have a bias for one polarization or the other because of their intrinsic birefringence. Birefringence is the decomposition of a ray of light into two rays (the ordinary ray and the extraordinary ray) when it passes through certain types of material.
Thus, there exists a need for a method of measuring the polarization state of the laser beam unbiased by receiver and by reflector would be convenient for depolarization studies near the ground. For example, the capability of distinguishing forward from backward scattering could be very useful in distinguishing the scattering of nonspherical dust particles from multiscattering of water droplets. There also exists a need for the ability to measure the forward scatter of a transmitter beam independently of the backscatter in real-world environments.